US20120299439A1 - Micro-electro-mechanical transducers - Google Patents

Abstract

Some examples include at least one capacitive micro-electro-mechanical transducer (cMUT). For instance, the cMUT may include a substrate, a plate, and a resilient structure therebetween. In some examples, an integrated circuit may be formed on or integrated with the plate or other portion of the cMUT. Furthermore, in some examples, two cMUTs may be arranged in a stacked configuration. For instance, one cMUT may be operable for transmission, while a second cMUT may be operable for reception.

Description

CROSS-REFERENCES TO RELATED APPLICATIONS
• This application is a continuation of, and claims priority to, U.S. patent application Ser. No. 11/914,597, filed Nov. 16, 2007, which is a national stage application of, and claims priority to, International Application No. PCT/IB2006/051568, filed May 18, 2006, which claims the benefit of U.S. Provisional Application Ser. No. 60/682,619, filed May 18, 2005; Ser. No. 60/692,038, filed Jun. 17, 2005; Ser. No. 60/705,606, filed Aug. 3, 2005; and Ser. No. 60/744,242, filed Apr. 4, 2006. All of the above-listed applications are incorporated herein by reference in their entirety.
• This application further incorporates herein by reference in entirety the following:
• International Application (PCT) PCT/IB2006/051566, entitled THROUGH-WAFER INTERCONNECTION, filed on May 18, 2006;
• International Application (PCT), PCT/IB2006/051567, entitled METHODS FOR FABRICATING MICRO-ELECTRO-MECHANICAL DEVICES, filed on May 18, 2006; and
• International Application (PCT), PCT/IB2006/051569, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS, filed on May 18, 2006.
FIELD OF THE INVENTION
• The present invention relates to micro-electro-mechanical devices that have a movable mechanical part for energy transformation, particularly to micromachined ultrasonic transducers (MUT) such as capacitance micromachined ultrasonic transducers (cMUT).
BACKGROUND OF THE INVENTION
• Micro-electro-mechanical transducers usually share a common feature, which includes a movable mechanical part used for energy transformation. One example of such micro-electro-mechanical transducers is micromachined ultrasonic transducers (MUT). An ultrasound transducer performs a chain of energy transformation to realize its function of a transducer. In its receiving mode, the acoustic energy of ultrasound waves propagating in a medium where the transducer is placed is transformed to mechanical energy of a movable part (conventionally a vibrating membrane) in the transducer. The motion of the movable part is then transformed to a detectable electromagnetic (usually electrical) signal. In its transmitter mode, the reverse chain of energy transformation takes place.
• Various types of ultrasonic transducers have been developed for transmitting and receiving ultrasound waves. Ultrasonic transducers can operate in a variety of media including liquids, solids and gas. These transducers are commonly used for medical imaging for diagnostics and therapy, biochemical imaging, non-destructive evaluation of materials, sonar, communication, proximity sensors, gas flow measurements, in-situ process monitoring, acoustic microscopy, underwater sensing and imaging, and many others. In addition to discrete ultrasound transducers, ultrasound transducer arrays containing multiple transducers have been also developed. For example, two-dimensional arrays of ultrasound transducers are developed for imaging applications.
• Compared to the widely used piezoelectric (PZT) ultrasound transducer, the MUT has advantages in device fabrication method, bandwidth and operation temperature. For example, making arrays of conventional PZT transducers involves dicing and connecting individual piezoelectric elements. This process is fraught with difficulties and high expenses, not to mention the large input impedance mismatch problem presented by such elements to transmit/receiving electronics. In comparison, the micromachining techniques used in fabricating MUTs are much more capable in making such arrays. In terms of performance, the MUT demonstrates a dynamic performance comparable to that of PZT transducers. For these reasons, the MUT is becoming an attractive alternative to the piezoelectric (PZT) ultrasound transducers.
• Among the several types of MUTs, the capacitive micromachined ultrasonic transducer (cMUT), which uses electrostatic transducers, is widely used. Other MUTs using piezoelectric (pMUT) and magnetic (mMUT) transducers are also adopted. Examples of prior art cMUT structure are shown inFIGS. 1A-1C,FIGS. 2-3, andFIGS. 4A-4B.
• FIG. 1A shows a cross-sectional view of a basic structure of a prior art cMUT having multiple cells.FIG. 1B shows an enlarged view of asingle cMUT cell10.FIG. 1C shows a corresponding schematic top view of the same prior art multi-cell cMUT structure. In practice, a functional cMUT may have at least one independently addressable cMUT element. Based on the conventional design, each cMUT element consists of many cMUT cells, which are connected in parallel. Four cells are shown inFIG. 1A, a single cell is shown inFIG. 1B, and ten cells are shown inFIG. 1C, but all cells belong to a single cMUT element inFIGS. 1A-1C.
• The cMUT ofFIGS. 1A-1C is built on asubstrate11. As shown in a selectedcMUT cell10, each cMUT cell has a parallel plate capacitor consisting of arigid bottom electrode12 and atop electrode14 residing on or within aflexible membrane16 that is used to transmit or receive an acoustic wave in the adjacent medium. Theflexible membrane16 in each cell is supported by the insulation wall orposts18. Themembrane16 is spaced from thesubstrate11 and thetop electrode12 to define a transducingspace19 therebetween. A DC bias voltage is applied between theelectrodes12 and14 to deflect themembrane16 to an optimal position for cMUT operation, usually with the goal of maximizing sensitivity and bandwidth. During transmission, an AC signal is applied to the transducer. The alternating electrostatic force between the top electrode and the bottom electrode actuates themembrane16 in order to deliver acoustic energy into the medium (not shown) surrounding the cMUT. During reception, the impinging acoustic wave vibrates themembrane16, thus altering the capacitance between the two electrodes. An electronic circuit detects this capacitance change.
• Alternatively, the membrane can be actuated and the displacement of the membranes detected using a piezoelectric transducer (pMUT) and a magnetic transducer (mMUT).FIG. 2 shows apMUT cell20, which has a similar structure to thecMUT cell10 except that the capacitor (electrodes12 and14) are replaced by apiezoelectric member24 on themembrane26.FIG. 3 shows anmMUT cell30, which has a similar structure to thecMUT cell10 except that the capacitor (electrodes12 and14) are replaced by amagnetic member34 on themembrane36.
• Methods of fabrication for making a cMUT shown inFIGS. 1A-1C have been developed. Exemplary methods are disclosed in U.S. Pat. Nos. 6,632,178 and 6,958,255.
• There are drawbacks in the cMUTs of the prior art structures and methods. Many of these drawbacks relate to the fact that each addressable cMUT element is made of many individual cells and each cell has its cMUT membrane clamped or fixed on edges shared by the adjacent cells. Examples of the drawbacks are listed below.
• (1) The average displacement of the membranes is small because of the clamped edges. As a result both the device transmission and reception performance are poor.
• (2) Surface areas occupied by the clamped areas (e.g., edges) and the walls or posts are non-active, thus reducing the device fill factor and the overall efficiency.
• (3) Anchor areas introduce a parasitic capacitance, which decreases the device sensitivity.
• (4) The anchor pattern within the surface of the cMUT element may cause ultrasonic wave interference, which limits the device bandwidth.
• (5) The non-uniform displacement of the membrane may disturb the ultrasonic wave pattern. For example, the non-uniform displacement may affect the ultrasonic beam pattern emitted from the transducer surface and also cause acoustic cross coupling through the transducer surface.
• (6) The resonant frequencies of individual cells in the same cMUT element may be different between each other because of the process variation. This causes phase differences of the membrane motion among different cells in the same cMUT element during operation. As a result, the sum of the average displacement of the cMUT element may degrade dramatically. This problem degrades the device performance especially when the cMUT works in a high quality factor (Q-factor) condition, for example in air.
• (7) The acoustic energy can couple into the transducer substrate through supporting walls and cause undesired effects such as acoustic cross coupling between the cMUT elements. An effort to reduce the cross coupling through the substrate by introducing materials with desired acoustic properties may require occupation of extra space between elements.
• The above problems also exist in the pMUT and mMUT of the prior art since they have a similar structure as the cMUT as shown inFIG. 1.
• Another cMUT device having a compliant support structure built on the substrate to support the membrane is disclosed in the U.S. Pat. No. 7,030,536. A cMUT according to that design is shown inFIGS. 4A-4B.FIG. 4A shows cross-sectional view of asingle cMUT cell40 as disclosed in that patent.FIG. 4B shows the schematic top view of multiple cMUT cells as disclosed in that patent. Compared to the conventional cMUT structure shown inFIGS. 1A-1C, the cMUT structure disclosed in U.S. Pat. No. 7,030,536 uses acompliant support structure48 in place of theconventional insulation wall18 to define the perimeter ends of themembrane46 of eachcMUT cell40, such that thetop electrode44 and themembrane46 may move in a piston-like manner. This has potential advantages but the design according to the patent also introduces its own problems, as will be discussed in view of the present invention in the detailed description.
• Due to the importance of these MUT devices, it is desirable to improve the technology in terms of performance, functionality, and manufacturability.
SUMMARY OF THE INVENTION
• This application discloses a micro-electro-mechanical transducer (such as a cMUT) that has a movable mechanical part to transform energy. The transducer has a substrate, a top plate, and a resilient structure therebetween. The resilient structure has multiple connectors distributed over the device element area to vertically transport the top player with distributed support not limited to edges. The resilient structure may be cantilevers, or cantilever-containing structures such as bridges or crossbars, formed using a middle spring layer covering cavities on the substrate. Connectors define a transducing space below the top plate. The resilient structure such as cantilevers enable a vertical displacement of the connectors, which transports the top plate in a piston-like motion to change the transducing space and to effectuate energy transformation. No separate cells are necessary for each addressable transducer element. Multiple device elements can be made on the same substrate.
• According to one aspect of the invention, a micro-electro-mechanical transducer is characterized in that:
• (a) the resilient structure has a lower portion connecting to the substrate and an upper portion connecting to the top plate layer;
• (b) the upper portion of the resilient structure has a plurality of connectors connecting to the top plate layer;
• (c) the plurality of connectors are distributed over the device element area and have at least one connector located substantially away from the device element periphery toward an inner region of the device element area;
• (d) a transducing space is defined below the top plate layer;
• (e) the addressable device element has at least one transducing member to induce energy transformation; and
• (f) the resilient structure enables a vertical displacement of the plurality of connectors to transport the top plate layer in a substantially vertical direction, thus changing the transducing space and activating the transducing member.
• In one embodiment, the resilient structure has a middle spring layer placed over the substrate. The middle spring layer and the substrate define at least one cavity inside the device element area. The cavity divides the device element area into at least two opposing sides, at least one side having an anchor anchoring the middle spring layer. The plurality of connectors are each horizontally distanced from the anchor by a sufficient length to define a cantilever or a bridge between the connector and the anchor. In this embodiment, each connector may have a desired height and stand from the top of the middle spring layer to separate the top plate from the middle spring layer to define the transducing space below the top plate layer.
• In one preferred embodiment, the substrate and the middle spring layer define a plurality of cavities dividing the device element area into segments each substantially smaller than the device element area. Multiple cantilevers or bridges may be defined over the plurality of cavities to provide a distributed support to the top plate layer.
• In one embodiment, the cavities and the corresponding anchors together occupy at least half the device element area. Within the device element area, the middle spring layer may be a contiguous layer covering the top surface of the anchor.
• In one embodiment, the contiguous middle spring layer covers at least half of the device element area, and preferably, the contiguous middle spring layer covers substantially the entire device element area.
• In one embodiment, the cavity has an annular shape dividing the substrate wafer into an inner island and an outer region, and the inner island constitutes the anchor. The anchor may include a first anchor comprising the inner island and an opposing second anchor comprising the outer region.
• In one embodiment, the cavity comprises a first plurality of extended cavities forming a pattern of cavities and anchors on the substrate wafer. Various patterns may be formed.
• The cantilevers may be of a variety of formations. For example, the cavity may be defined by at least two anchors opposing each other, and the middle spring layer within the device element area may connect to both opposing anchors. The connector may be located between the two opposing anchors and horizontally distanced from each of the two opposing anchors by a sufficient length to define two cantilevers connected head-to-head at a location where the connector is positioned. For another example, the anchor may define a circular sidewall surrounding the cavity, the middle spring layer may cover the entire cavity, and the connector may be at a position directly above a middle area of the cavity to define a two-dimensional plane cantilever.
• One aspect of the invention is a micro-electro-mechanical transducer wherein the middle spring layer has a plurality of connectors connecting to the top plate layer. The plurality of connectors is distributed across the device element area to define a connector density profile having an average connector density per area location. Each cantilever thus formed has a spring strength, which when combined with the respective average connector density defines an effective spring intensity profile in which the effective spring intensity is the spring strength per area location. The effective spring intensity may either be substantially uniform over the device element area or have a nonuniform distribution profile. For example, the effective spring intensity may be significantly higher at area locations near a center of the device element area than locations near a perimeter of the device element area.
• In one embodiment having multiple connectors, the device element area comprises two regions having different thicknesses of the top plate layer and different spring intensities. For example, a first region may have a greater thickness of the top plate layer and great effective spring intensity than the second region, or vice versa. In another embodiment, the device element area comprises a perimeter surrounding a middle region, wherein the top plate layer is flexible near the parameter but rigid in the middle region, clamped at a perimeter sidewall and supported by at least one cantilever in the middle region.
• The micro-electro-mechanical transducer may have additional characteristics. For example, the addressable device element the top plate layer may have a thickness profile having at least two different thicknesses across the device element area. The two different thicknesses may comprise a first thickness corresponding to a normal thickness of the top plate layer and a second thickness corresponding to a bottom thickness of a cavity formed on the top plate layer, the first thickness and the second thickness regularly alternating across the device element area. For another example, the addressable device element the top plate layer may comprise a first material and a second material different from the first material. In one embodiment, the first material has a plurality of segments divided by separation trenches and the second material spans over the segments to connect the segments of the first material. The second material may also at least partially fill the separation trenches.
• In one embodiment, the micro-electro-mechanical transducer is a capacitance micromachined ultrasonic transducer (cMUT), wherein each addressable device element has a top electrode on the top plate layer and a bottom electrode on either the middle spring layer or the substrate wafer. Example, the middle spring layer may comprise a conductive material to effectuate as the bottom electrode. Alternatively, the top electrode may be a separate conductive layer deposited on the top plate layer.
• In another embodiment, the micro-electro-mechanical transducer has a plurality of addressable device elements, which may be separated by a separation trench formed through at least part of the top plate layer. The substrate wafer may be a conductive wafer, and the addressable device element is insulated by an embedded insulator formed on the substrate wafer. The embedded insulator comprises an insulation cavity having a desired overall thickness and being filled with a dielectric material. Alternatively, the addressable device element is insulated by an embedded insulator formed on the substrate wafer, the embedded insulator comprising a patterned cavity containing solid lines of oxidized native material of the substrate wafer. The embedded insulator may also be formed on the top plate layer.
• In one embodiment, the addressable device element is addressed from the back side of the substrate wafer via a through-wafer interconnect formed through the substrate wafer. For example, a conductive substrate wafer may be used, and the through-wafer interconnect may include an annular trench surrounding a through-wafer conductor made of a native material of the conductive wafer. A dielectric material may be used to fill the annular trench to insulate the through-wafer conductor from the rest of the substrate wafer. In one particular configuration, the through-wafer conductor is positioned directly underneath the addressable device element.
• The micro-electro-mechanical transducer may also be a pMUT, wherein the transducing member comprises a piezoelectric member disposed on the middle spring layer or the top plate layer. The micro-electro-mechanical transducer may also be a mMUT, wherein the transducing member comprises a magnetic member disposed on the middle spring layer or the top plate layer.
• The performance of the micro-electro-mechanical transducer may be enhanced by careful selection of the top plate material or configuration. For example, the top plate layer may have holes formed therethrough. The holes may have a size and location profile that optimizes a rigidity/mass ratio of the top plate layer. Furthermore, in order to avoid electric shorting problem, the maximum vertical displacement of the connector may be limited to be no greater than, preferably less than, the height of the transducing space between the top plate layer and the top surface of the anchor including any intervening layer. In one embodiment, the maximum vertical displacement of the connector is limited by a motion stopper disposed in the cavity.
• According to another aspect of the invention, a micromachined ultrasonic transducer (MUT) structure has a substrate wafer, a middle spring layer and the top plate layer arranged in that order such that a bottom side of the top plate layer faces a top side of the middle spring layer and a bottom side of the middle spring layer faces a front side of the substrate wafer. The MUT structure comprises a plurality of addressable MUT elements each defined by a MUT element area of the substrate wafer and a corresponding MUT element area of the top plate layer. Each of the plurality of addressable MUT element has a transducing member and is further characterized in that:
• (a) the device element area has at least one cavity between the substrate and the middle spring layer, each cavity being sidewalled by at least one anchor anchoring the middle spring layer;
• (b) the middle spring layer covers at least a part of each cavity;
• (c) at least one connector of a desired height disposed above each cavity, the connector standing from the top of the middle spring layer;
• (d) the top plate is placed over the connector(s) to define a transducing space below the top plate layer;
• (e) each connector is horizontally distanced from one of the at least one anchor by a sufficient length to define a cantilever between the connector and the anchor, wherein the cantilever and the cavity enable a vertical displacement of the connector to transport the top plate layer substantially vertically with a piston-like motion, thus changing the transducing space and activating the transducing member; and
• (f) the at least one cavity and the corresponding anchor(s) occupy at least half of the MUT element area.
• The MUT structure may have additional features as summarized in above.
• The foregoing and other features and advantages will become more apparent from the following detailed description of several embodiments, which proceeds with reference to the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1A shows a cross-sectional view of a basic structure of a prior art cMUT having multiple cells.
• FIG. 1B shows an enlarged view of a single cMUT cell ofFIG. 1A.
• FIG. 1C shows a corresponding schematic top view of the same prior art multi-cell cMUT structure.
• FIG. 2 shows a prior art pMUT cell.
• FIG. 3 shows a prior art mMUT cell.
• FIG. 4A shows cross-sectional view of another broiled single cMUT cell.
• FIG. 4B shows a corresponding schematic top view of multiple cMUT cells ofFIG. 4A.
• FIG. 5A shows a cross-section view of a cMUT structure in accordance with the present invention showing a complete cMUT element and parts of neighboring cMUT elements.
• FIG. 5B shows a schematic top view of the cMUT element shown inFIG. 5A without showing the top plate.
• FIG. 5C shows an enlarged view of a selected cMUT portion of the cMUT structure ofFIG. 5A.
• FIG. 5D shows an enlarged view of a different selected cMUT portion, which is another part of the complete cMUT element inFIG. 5A.
• FIG. 6 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a first example of an addressable cMUT element in accordance with the present invention.
• FIG. 7 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a second example of an addressable cMUT element in accordance with the present invention.
• FIG. 8 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a third example of an addressable cMUT element in accordance with the present invention.
• FIG. 9 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fourth example of an addressable cMUT element in accordance with the present invention.
• FIG. 10 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fifth example of an addressable cMUT element in accordance with the present invention.
• FIG. 11 shows a cross-section view of another cMUT structure of the present invention showing a complete cMUT element and parts of neighboring cMUT elements.
• FIGS. 12A-12J show examples of spring (cantilever) distribution profiles.
• FIG. 13 shows an example of doping profile of a silicon middle spring layer.
• FIG. 14 shows an enlarged view of a selected portion of a cMUT element of the present invention having a motion stopper to limit the maximum vertical displacement of the top plate.
• FIGS. 15A-15C show three exemplary configurations of the top plate layer.
• FIG. 15D shows a graph of the corresponding ratio of 1st resonant frequency over the total mass of the top plate as the function of the diameter of the etched holes shown in the three configurations.
• FIG. 15E shows another example of the top plate having holes built therein.
• FIG. 16 shows the cross-section view of a section of a pMUT element according to an exemplary embodiment of the present invention.
• FIG. 17 shows the cross-section view of a section of a mMUT element according to an exemplary embodiment of the present invention.
• FIG. 18 shows a cross-section view of a cMUT device having a cMUT integrated with an IC in accordance with the present invention.
• FIGS. 19A-19D illustrate an example of fabrication of a cMUT device having ICs integrated with a cMUT transducer element according to some implementations.
• FIGS. 20A-20B illustrate examples of cMUT transmission and reception operation according to some implementations.
• FIGS. 21A-21B illustrate examples cMUT devices having a first cMUT and a second cMUT according to some implementations.
• FIG. 22A illustrates a cross-section view of a cMUT device having a first cMUT integrated with a second cMUT according to some implementations.
• FIG. 22B illustrates a cross-section view of the cMUT device ofFIG. 22A further incorporating integrated circuits according to some implementations.
• FIG. 23A illustrates a cross-section view of a cMUT device having a first cMUT integrated with a second cMUT according to some implementations.
• FIG. 23B illustrates a cross-section view of the cMUT device ofFIG. 23A further incorporating integrated circuits according to some implementations.
DETAILED DESCRIPTION
• The micro-electro-mechanical transducer such as a capacitance micromachined ultrasonic transducer (cMUT) of the present invention will be described in detail along with the figures, in which like parts are denoted with like reference numerals or letters. The micro-electro-mechanical transducer may be fabricated using any suitable methods, particularly using the methods disclosed in several other patent applications identified herein, which are filed by a common applicant on even date.
• The invention has been described below with reference to specific embodiments. In most cases, a cMUT structure is used to illustrate the invention. It is appreciated, however, that the present invention is not limited to cMUTs. It will be apparent to those skilled in the art that various modifications may be made and other embodiments can be used without departing from the broader scope of the inventions. Therefore, these and other variations upon the specific embodiments are intended to be covered by the present inventions. Those skilled in the art will recognize that various features disclosed in connection with the embodiments may be used either individually or jointly.
• It is noted that the terms “transducer” and “transducing member” are used in a broad sense in this document to not only include devices that perform both actuation and sensing functions but also include devices that perform either an actuation function or a sensing function. It is also noted that the term “cantilever” is used in this description in a broad sense to describe a structure that has an anchored end, a resilient portion extending from the anchored, and to an exerting end to activate or move the resilient portion. A cantilever thus does not necessarily suggest a literal one-dimensional bema-like cantilever, but also includes similar structures have multibeams extending in different directions such as a bridge, or a crossbar, and most definitely also includes area or plane springs (two-dimensional “cantilevers”) in which the anchored end is an extended line which may be a closed perimeter of an area or a portion thereof, the resilient portion is an extended area, and the exerting end may be a single point, a small area, or an extended line (close ended, open-ended, or segmented). In addition, the words “circular” and “annular” only suggest in the broadest sense that a shape has a looped form, a curved shape that is nearly looped, or an arrangement that is generally shaped like a ring, and do not suggest a rounded shape or any other shape in particular, nor does it suggest that the loop or ring is entirely complete or unbroken.
• In order to illustrate the present invention, certain aspects of the designs according to the prior art are first discussed in light of the present invention. It is noted that the discussion herein casts a hindsight on the prior art designs in light of the present invention for the purpose of clearer illustration.
• Referring toFIGS. 4A and 4B, a prior art cMUT design is shown to use acompliant support structure48 in place of theconventional insulation wall18 to define the perimeter ends of themembrane46 for eachcMUT cell40, such that thetop electrode44 and themembrane46 may move in a piston-like manner through thespace45 between thebottom electrode42 and thetop electrode44. Thetop electrode44 is attached to bottom of themembrane46. Thebottom electrode42 sits on top of apedestal43. In each cMUT cell, themembrane46 is fixed to thecompliant support structure48 which is essentially a sealed sidewall to constitute the perimeter48 (FIG. 4B) of eachcMUT cell40 the same way as the fixed membrane-based cMUT design showing inFIGS. 1A-1C. This is better seen in the context of the multi-cell structure shown inFIG. 4B.
• The above design has potential advantages but also introduces its own problems. First, it should be noted that the concept of the prior art cMUT design shown inFIGS. 4A-4B is to replace the conventionalrigid cell sidewall18 using acompliant cell sidewall48 made of compliant support structures. Because a relatively complexcompliant cell sidewall48 takes the place of the simple andnarrow insulation wall18 inFIG. 1, there would be a heightened challenge to minimize the impact of the inactive areas occupied by these peripheral compliant cell sidewalls48 according to that design. It has been suggested that supplemental electrodes be made on the compliant support structures in thecompliant cell sidewall48 to reduce the inactive areas occupied by the compliant support structures. There is however no indication that such a design would solve the problem, and further no indication whether the design is practical from a fabrication point of view.
• Second, thecompliant cell sidewall48 defines the boundary in the overall cMUT structure, as shown inFIG. 4B. The vast majority area of thecMUT cell40 is therefore occupied by a rigid andnonmovable pedestal43, which is surrounded by (but separated from) the peripheralcompliant cell sidewall48. Thecompliant cell sidewall48 on the edge is the sole structure to support theentire membrane46 in thecell40. This retains the same cell limitation that exists in the design of the cMUT showing inFIGS. 1A-1C. For example, it limits the overall size of eachsingle cMUT cell40. A large cMUT cell would require alarge membrane46, which would have to be very rigid and thick in order to maintain the required resonant frequency. This is one of the most important limitations of the conventional cMUT structure, because of which each addressable cMUT element must be made of multi-cells. For example, in a cMUT structure used in medical phase array, the cMUT element size is the half wavelength of the acoustic wave (e.g., 75 um for a 10 MHz device, 150 um for a 5 MHz device and 750 um for a 1 MHz device). In order to achieve the required device operation frequency, the size of the conventional cMUT cells must be made much smaller than the element or device size without having to use an unreasonably thick membrane. Due to its inherent design concept, the cMUT shown inFIGS. 4A and 4B is likely to behave quite similar to the other cMUT structures in this respect and therefore does not promise to solve the related problems. In fact, it was envisioned that multiple small cells similar to what is found in the cMUT structures shown inFIGS. 1A-1C are used for the cMUT structures inFIGS. 4A and 4B.
• The present invention is envisioned to do away with the cMUT cell requirement. As will be shown next, although the present invention may still be used to fabricate cMUT elements each having multiple cells, it does not have to be. In fact, due to the inherent characteristics of the cMUT design in accordance with the present invention, it may be preferable from a fabrication point of view to make each addressable cMUT element without any internal cell boundaries. In addition, even when an addressable cMUT element in accordance with the present invention is made of multiple smaller segments (such as areas each having its own top plate layer segment and middle spring layer segment separated from that of others), these smaller segments do not need to have a clamped perimeter or a peripheral support wall, and further do not need to be identical to each other in size or shape.
• FIG. 5A is a cross-section view of a cMUT structure showing acomplete cMUT element500 and parts of neighboringcMUT elements500A and500B, one on each side. The cMUT structure is built on asubstrate wafer501 and also has amiddle spring layer520 and atop plate540. Thesubstrate wafer501, themiddle spring layer520 and thetop plate layer540 are arranged from bottom to top in that order such that the bottom side of thetop plate layer540 faces the top side of themiddle spring layer520, and the bottom side of themiddle spring layer520 faces the front side of thesubstrate wafer501. ThecMUT elements500,500A and500B are separated byseparation trenches515 formed through thetop plate540 and themiddle spring layer520.
• In this description, a cMUT element or a micro-electro-mechanical element refers to a device unit that is capable of performing an energy transformation through the control of an external or built-in control circuit and is individually addressable through the external or built-in control circuit. Each addressable cMUT element defines a device element area of the substrate wafer and a corresponding device element area of the top plate layer. Because the three layers (thesubstrate wafer501, themiddle spring layer520 and the top plate layer540) are arranged substantially parallel to each other and stacked directly on top of each other, the device element area of the substrate wafer and the corresponding device element area of the top plate layer overlie each other vertically and coincide with each other longitudinally (over the surface dimension of the layers).
• In one aspect of the present invention, a resilient structure (such as themiddle spring layer520 in the example shown inFIG. 5A) having multiple connectors distributed over the device element area is provided to vertically transport the top player. The multiple connectors facilitate distributed support not limited to edges (peripheral areas) only. In the exemplary structure shown inFIG. 5A, for example, thetop plate540 is connected to themiddle spring layer520 through multiple plate-spring connectors530,530aand530b. Althoughconnectors530bare located near the device element peripheral defined byseparation trenches515,other connectors530 and530aare substantially away from the device element peripheral toward the center of the device element area to provide a more flexible, more efficient, and more distributed support to thetop plate layer540. This distributed support may help build a cMUT element of a larger size.
• For a cMUT that has a very high operating frequency cMUT, however, a basic unit shown inFIGS. 5C-5D may be used as a full cMUT device or element.
• FIG. 5B is a schematic top view of thecMUT element500 without showing thetop plate540. In cross-sectional view of thecMUT element500 taken along broken line indicated inFIG. 5B is shown inFIG. 5A. In this exemplary configuration, thecMUT element500 is patterned with concentric squares. In the center is a simplediscrete connector530 positioned on top of thecavity502 of a simple shape (as shown in the cross-sectional view inFIG. 5A theconnector530 is placed on themiddle layer520 which covers the cavity502), which is surrounded by acircular sidewall anchor503, which in turn is surrounded by acircular cavity502a. Another largercircular connector530ais positioned on top of thecircular cavity502a, which is surrounded by acircular sidewall anchor503aof the next level. As will be shown inFIGS. 6-10, however, a variety of different configurations are possible with the present invention.
• FIG. 5C is an enlarged view of a selectedcMUT portion510, which is a part of thecomplete cMUT element500. The selectedcMUT portion510 is a basic unit, of the complete cMUT element500 (andother cMUT elements500A and500B). The structure of the selectedcMUT portion510 provides a basis to understand thecomplete cMUT element500.
• As shown inFIG. 5C, the selectedcMUT portion510 includes two halves centered on an imaginary vertical line (not shown) going through the structure from the top to the bottom. As mentioned, for certain application such as a cMUT with a high operation frequency, a full cMUT element or device may use only onecMUT portion510. For other applications, it may be preferred to use a combination of multiple basic unit is shown inFIG. 5C andFIG. 5D.
• The basic structure unit of the cMUT element is built on asubstrate wafer501 on top of which there is a standing feature (referred to as “sidewall anchor” hereinafter)503 having two sidewalls on two opposingsides bordering cavities502 and502a, respectively. The standing feature (sidewall anchor)503 may be an integrated part of thesubstrate501 formed as a result of forming thecavities502 and502a, but may also be an additional structure added onto a separate substrate. In one embodiment, for example, thesidewall anchor503 is part of themiddle spring layer520. The substrate of 501 may be made either of a nonconductive material or a conductive material such as silicon or polysilicon. In a configuration where thesidewall anchor503 is a separate structure, conductivity of thesidewall anchor503 may be the same as or different from that of thesubstrate501. For example, thesubstrate501 may be made of a nonconductive material while thesidewall anchor503 a conductive material such as silicon or polysilicon.
• The cMUT structure shown also has thesecond cavity502along the other side ofsidewall anchor503. Depending on how and where thecMUT portion510 is taken from thecMUT element500, thesecond cavity502amay either belong to a different and separate cavity, or just another portion of a same circular or extended cavity as thecavity502. The selectedcMUT portion510 also has asecond connector530ain the other half. Again, depending on how and where thecMUT portion510 is taken from thecMUT element500, thesecond connector530amay either be a part of a different and separate connector, or just another portion of a same circular or extended connector as theconnector530.
• ThecMUT structure portion510 further has these components: amiddle spring layer520 which is preferably an elastic membrane, abottom electrode525 placed on themiddle spring layer520, aconnector530 which stands on top of themiddle spring layer520, aninsulation layer535 sitting over theconnector530, atop plate540 connected to theconnector530 through an interveninginsulation layer535, and atop electrode550.
• Theconnector530 stands out from themiddle spring layer520 to define atransducing space560 below thetop plate540. The transducingspace560 is generally defined between thetop plate layer540 and the top surface of themiddle spring layer520 or the top surface of thesidewall anchor503, whichever is higher. Where there is an intervening layer between thetop plate layer540 and the top surface of themiddle spring layer520 or the top surface of thesidewall anchor503, the available transducing space may be reduced. For example, if another layer is deposited over thesidewall anchor503, and yet another layer is deposited on the bottom of thetop plate540, the transducing space would be defined between these two intervening layers. In the exemplary configuration shown inFIG. 5C, the actual height of the available transducingspace560 may be reduced by the thicknesses of theinsulation layer535, thebottom electrode525 and themiddle spring layer520. It should be noted that in some embodiments it is possible to have the entire height between thetop plate layer540 and the top surface of thesidewall anchor503 available for the transducingspace560. For example, a conductive substrate wafer may itself be used to effectuate a bottom electrode on the substrate (e.g., on the sidewall anchor503) without requiring a separate electrode layer; and cantilevers may be made with segments of middle spring layers connected to thesidewall anchor503 at sides flush with or lower than the top surface of thesidewall anchor503, instead of using a continuous middle spring layer placed on top of thesidewall anchor503.
• In some embodiments, bothsubstrate501 including thesidewall anchor503 and themiddle spring layer520 are conductive. In this case, thesubstrate501 may serve as a conductor to access the conductivemiddle spring layer520, while themiddle spring layer520 may serve as the bottom electrode.
• Theconnector530 is horizontally distanced from thesidewall anchor503 by a sufficient length to define a cantilever anchored at thesidewall anchor503 with an exertingend522 on theconnector530. The cantilever and thecavity502 enable a vertical displacement of theconnector530, which transports thetop plate540 substantially vertically with a piston-like motion, thus changing the transducingspace560. When the both halves of thecMUT structure510 move in the same phase, the vertical piston-like motion is further assured.
• In this particular example shown, top surface of thesidewall anchor503 is covered by themiddle spring layer520, which in turn is covered by thebottom electrode525. Furthermore, thetop plate540 and theconnector530 do not connect with each other directly but are intervened by theinsulation layer535 therebetween. The transducingspace560 between thetop plate540 and the top surface of thesidewall anchor503 is therefore partially occupied by themiddle spring layer520, thebottom electrode525 and theinsulation layer535. It is noted that the part of themiddle spring layer520 covering the top surface of thesidewall anchor503, thebottom electrode525 and theinsulation layer535 are optional. In any event, in order to achieve the intended energy transformation, the transducingspace560 should not be entirely occupied by these extra layers if they are included in the structure.
• FIG. 5D is an enlarged view of a different selectedcMUT portion511, which is another part of thecomplete cMUT element500. Compared to the selectedcMUT portion510 shown inFIG. 5C, the selectedcMUT portion511 is taken from a shifted location. The selectedcMUT portion511 is built on asubstrate501, which has acavity502 bordered by two sidewall anchors503 and503aon two opposite sides. ThecMUT structure portion511 further has these components:middle spring layer520,bottom electrode525 placed onmiddle spring layer520,connector530 which stands on top of themiddle spring layer520,insulation layer535 sitting over theconnector530,top plate540 connected to theconnector530 through an interveninginsulation layer535, andtop electrode550.
• The connector530 (which is also shown inFIG. 5C) stands on the middle spring layer, and is horizontally distanced from the sidewalls of both thesidewall anchor503 and thesidewall anchor503a. Themiddle spring layer520 between thesidewall anchor503 and thesidewall anchor503adefines a double-cantilever anchored at thesidewall anchor503 and thesidewall anchor503a. The double-cantilever is connected head-to-head atlocation522 where theconnector530 is located to form a bridge.
• Thetop plate540 is placed over theconnector530, which separates thetop plate540 from themiddle spring layer520 to define a transducing space below the top plate. The double-cantilever and thecavity502 enable a vertical displacement of theconnector530, which transports thetop plate540 substantially vertically, thus changing the transducing space and activating a transducing member in the transducer for energy transformation.
• The above novel cMUT design has essentially done away with the conventional concept of the cell insulation wall which divides a cMUT element into cells and is required to support and clamp the membrane at the perimeter of each cMUT cell. The conventional cMUT designs shown inFIG. 1, including the cMUT design as disclosed in the U.S. Pat. No. 7,030,536, all require a cell insulation wall (e.g.,cell insulation wall18 inFIGS. 1A-1C, or the compliantcell insulation wall48 inFIGS. 4A-4B) at the perimeter of each cMUT cell to support and clamp the perimeter of the membrane. The cell insulation wall defines the prior art cMUT cells. Within the perimeter defined by the insulation wall, the bottom electrode in the prior art cMUT structure is either directly deposited on the substrate11 (FIG. 1) or deposited on the top of a pedestal43 (FIG. 4A), which is separated from the cell insulation wall. Because the majority of the membrane (16 inFIG. 1 or46 inFIG. 4A) is unsupported, the size of each cell and the selection of the materials for the membrane is limited.
• In contrast, with the cMUT design as shown inFIG. 5A andFIG. 5B, the cMUT element is no longer required to be divided into cells and thus has no need of an insulation wall to define cell perimeters. Thetop plate layer540 and thetop electrode550 are supported by a resilient structure (multiple cantilevers in the embodiment shown) through multiple connectors that can be freely positioned and distributed based on need, thus effectively distributing the entire load of thetop plate layer540 over multiple springs (cantilevers). This solves the problem of the cell limitation inherent to the prior art designs. Themiddle spring layer520 and abottom electrode525 are supported by multiple sidewall anchors503, which can also be distributed across theentire substrate wafer501 based on need, rather than being limited to peripheral areas. There is no central pedestal and a separate peripheral compliant support structure. Only multiple sidewall anchors503 distributed across the entire device element area are required to support themiddle spring layer520 and thebottom electrode525. Eachanchor503 serves both as a support to the bottom electrode and as an anchor to a corresponding cantilever (or two cantilevers in the configuration shown inFIG. 5A andFIG. 5B, one on each side of the sidewall anchor503).
• With this design, a cMUT element with a very large active area may be formed. The operating frequency or frequency response of the cMUT element may be adjusted not only by selection of the materials for thetop plate540 but also by the configuration of the multiple cantilevers, including the spring strength of individual cantilevers and the cantilever distribution density profile across the area of the cMUT element.
• In principle, a cMUT element of the present invention may have an active area significantly larger than what would be possible with the conventional cMUT structures. The active area of a cMUT may be defined as the total movable area. For cMUT elements of a comparable overall size (cMUT element area), the active area of the cMUT element of the present invention may still be much greater than the total active area of the multiple cMUT cells in a conventional cMUT element. For example, the active area of the inventive cMUT is close to 100% of the element (device) area, while the total active area of multiple cMUT cells in a conventional cMUT element may cover about 60-90% of the element (device) area. Usually, the higher the operation frequency is, the lower the active area percentage is for a conventional cMUT. In addition, even for a given movable area, the cMUT of the present invention potential yields a greater effective active area, which may be defined as the area of the effective electric field activating the capacitors.
• The entiretop plate540 formed above thesubstrate wafer501 may be movable without any clamped or fastened area. If desired, multiple cMUT elements can be formed by formingseparation trenches515 through thetop plate540 and themiddle spring layer520. In principle, however, the entire cMUT structure with a very large active area may be used as a single cMUT element.
• Furthermore, with the cMUT structure design inFIG. 5A andFIG. 5B, thetop plate540 may be separated into a plurality of smaller top plates of identical or different sizes and shapes. Each smaller top plate may be addressed as a single cMUT element; alternatively, multiple smaller top plates may be combined together and addressed as a single cMUT element.
• Furthermore, unlike the flexible membrane clamped on its edges (or posts) in the conventional cMUTs, thetop plate540 shown inFIG. 5A andFIG. 5B can be designed to be either flexible or rigid. With a rigid top plate, the whole surface of the cMUT, which may include any number of separate smallertop plates540, may be movable with a very uniform displacement profile.
• The location of thetop electrode550 may be at any position above the transducingspace560. Thebottom electrode525 can reside on thesubstrate wafer501 and/or on a part of themiddle spring layer520. Thetop plate540 is actuated by the applied electrode field between twoelectrodes550 and525 to transmit the ultrasound into the medium, and the ultrasound can be detected if it impinges on thetop plate540 cause the capacitance between twoelectrodes550 and525 to change.
• In theexemplary cMUT structure500 shown inFIGS. 5A-5B, themiddle spring layer520 is a continuous layer placed over the top of the multiple sidewall anchors503 and amultiple cavities502. Themiddle spring layer520 thus occupies the entire device element area longitudinally (i.e., in the lateral or surface dimension). This configuration allows the entiretop plate540 to be supported by cantilevers throughconnectors530 that can be placed or distributed all over the entire device element area without being limited to places near the edge of the element. It is appreciated, however, that themiddle spring layer520 may comprise multiple small segments that are either connected to each other at certain points or completely separated from each other. Particularly, segments ofmiddle spring layer520 may be used in a way where each segment is connected or anchored to arespective sidewall anchor503 and covers at least part of arespective cavity502 to allow cantilevers to be formed, but leaves part of the top surface of the sidewall anchors503 uncovered. Such a configuration may allow the flexibility of using different materials and different thicknesses for themiddle spring layer520, but may complicate the fabrication process.
• Regardless of the configurations of the middle spring layer, it is envisioned that in some preferred embodiments, the cantilever-forming anchor surface(s) (the top surface of the sidewall anchors503) and the associated cantilever areas ofcMUT element500 together cover at least half of the device element area in order to achieve an effectively distributed cantilever-support of thetop plate540 and greater effective active areas. The cantilever areas may be defined as the total area of the cavities, or alternatively as the total area of themiddle spring layer520 covering the cavities. Preferably the cantilever-forming anchor areas and the cantilever areas are at least 80% of the device element area, and further preferably near 100% of the device element area, as is the case showing inFIG. 5A. Pedestals that do not serve to anchor a cantilever is not required, and preferably not formed at all within the device element area of thesubstrate wafer501 in order to maximize the cantilever-forming anchor areas as described above. In some embodiment, the cantilever areas and the associated connector areas are minimized while the cantilever-forming anchor areas are maximized for higher effective electrical field area for energy transformation. For a given number of the cantilevers, the cantilever area may be minimized by using a thinner middle spring layer with shorter cantilever dimensions while still achieving desired spring strength. These configurations provide a potential advantage over the prior art designs that have peripheral support for clamped membranes.
• Within the general principle of the present invention, there is a great room of design freedom in terms of the overall size of each addressable device element, the size, shape and arrangement of the cavities, the size, shape and arrangement of the anchors for cantilevers, the size, shape and arrangement of the connectors, and the thickness, shape, segment (separation) pattern and material selection of each layers (the substrate wafer, the middle spring layer and the top plate layer).
• A cMUT element in accordance with the present invention is no longer necessarily made of multiple cells each having a flexible membrane. Instead, the cMUT element may have a single (rigid or flexible) top plate. Even if multiple top plates are used, these top plates do not need to be clamped at some cell boundaries like the membranes for each cell in the prior art. The multiple top plates further do not need to be identical to each other. Generally, even with multiple top plates, only a small number of the top plates would be necessary for each addressable device element, much fewer than the number of individually clamped cells that would have been required in conventional cMUT designs.
• The cantilevers formed from the spring middle layer function as embedded springs that can be of a variety of spring strength, size and density variations. These embedded springs may be made of a single contiguous flexible membrane or multiple flexible membranes of identical or different size and shape. The locations of the plate-spring connectors (e.g., connectors530) may be designed to obtain the optimal displacement for the top plate or the desired frequency response for the cMUT during the cMUT operation. The configuration of thetop plate540, such as using smaller top plate segments, may also be adjusted to achieve desired frequency response for a cMUT element with a relative large size.
• In summary, unlike the cMUTs in the prior art, there may be no clamped (or fixed) area on the transducer surface of this invention, and the top surface of the cMUT may be made of a single top plate or multiple top plates which are supported by springs (cantilevers) deployed at strategic locations. As a result, the cMUT in accordance with the present invention has the potential to solve many problems of the prior cMUTs mentioned in the background section. The potential advantages may include:
• (1) Since the entire top surface of the transducer is movable with a much more uniform displacement, both the average displacement and the average electrical field between two electrodes of the cMUT element may be improved. This may lead to better transmission and reception performances.
• (2) With the design of the present invention, it is possible to make a transducer with no or very little inactive surface area so that the device fill factor can be near perfect. This is especially important for a high frequency cMUT because the transducer dimension is small.
• (3) The parasitic capacitance of the transducer can be dramatically decreased by selecting proper properties of the middle spring layer. For example, non-conductive materials may be used for the middle spring layer. This improves the transducer sensitivity.
• (4) The transducer breakdown voltage can be improved by using a proper material for the middle spring layer so that a high electrical field may be obtained between two electrodes to improve the device performance.
• (5) The uniformity of the cMUT surface displacement can be further improved by increasing the stiffness of the top plate. With a properly chosen thickness of the top plate, the cMUT surface displacement may have minimum impact or no impact on ultrasonic beam pattern emitted from the transducer surface.
• (6) Because the cMUT cell boundary pattern within the cMUT element surfaces can be eliminated with the new design, there is no high frequency limitation caused by such cell boundary patterns. This may improve the bandwidth of the cMUT.
• (7) Without having to use a great number of individual cells in each cMUT element, the phase difference of the motion at the different locations (different cells) on the cMUT surface is minimized. This is true especially if a rigid top plate is used. This can improve the device performance, especially when the cMUT works in a high quality factor (Q-factor) condition, e.g., in air or low pressure environment.
• (8) The anchors (sidewall anchors503 for anchoring the middle spring layer520) of the cMUT may be made slightly smaller than the top plate so that there is more room at the cMUT element edge for adding decoupling structures between the cMUT elements to reduce the cross coupling.
• (9) The present invention provides more design flexibility for the MUTs than those of the prior arts. For example, thetop plate540 may be designed to be of different shapes and configurations; the embedded springs (e.g. cantilevers made from the middle spring layer as shown inFIGS. 5A and 5B) may have different shapes and different spring constants by selecting the proper membrane dimensions. Moreover, the embedded springs can attach to the different locations on thetop plate540.
• FIGS. 6-11 show five exemplary designs of the spring (cantilever) distribution in accordance with the present invention.FIG. 6 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a first example of an addressable cMUT element in accordance with the present invention. Theaddressable cMUT element600 has an overall square shape, and includes:
• (1) aperipheral cavity602awith an annular shape defining the outer rim of thecMUT element600;
• (2) acircular connector630aplaced over theperipheral cavity602a;
• (3) anouter anchor603asurrounded by theperipheral cavity602a, theouter anchor603abeing contiguous throughout the general device element area of the substrate wafer except for the inner cut-out areas as shown;
• (4) four identicalinner cavities602beach having an annular shape, eachcavity602bdividing the substrate wafer into an inner island (respective anchor603b) and an outer region (the commonouter anchor603ain this particular example);
• (5) four identical innercircular connectors630beach placed over a respectiveinner cavity602b; and
• (6) four identicalinner anchors603bsurrounded by a respectiveinner cavity602b.
• In this example, theoutfit anchor603ais connected to thecircular connector630athrough a middle spring layer (not shown). The middle spring layer may be a single continuous layer covering the entire device element area, or separate areas connected to the anchors at selected positions. Although the middle spring layer is thin in its vertical dimension, it may be of any desired span or size on its surface (longitudinal or lateral dimension) up to covering the entire device element area. Preferably, the middle spring layer is contiguous over large areas to make plane cantilevers instead of separate narrow strip cantilevers. In the example shown inFIG. 6, because thecircular connector630ais a contiguous closed circle, a single plane cantilever may be formed over theperipheral cavity602a, anchored on the circular rim of theouter anchor603a. However, it is appreciated that theconnector630a(and other connectors herein) may be of any pattern, including separate segments. Similar formation possibilities apply to the fourconnectors630band the cantilevers formed over the fourcavities602b. It is appreciated that the shapes and relative sizes of thecavities602b,connectors630band theanchors603bcan be varied rather freely without departing from the general concept of the present invention. In particular, the term “circular” used in this description refers to a formation that has a substantially close-ended configuration, and does not suggest in any way the item or formation preferred to is round-shaped or in any other particular shape.
• Furthermore, the cantilevers formed overcavities602bmay be anchored either on theouter anchor603aor on theinner anchor603b, or both (to form a bridge-like double cantilever connected head to head).
• FIG. 7 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a second example of an addressable cMUT element in accordance with the present invention. Theaddressable cMUT element700 has an overall square shape, and includes:
• (1) a plurality ofcircular cavities702a,702b,702c, and702dof decreasing sizes each containing the trenches of smaller sizes (if there is any);
• (2) a plurality ofcircular connectors730a,730b,730cand730dplaced over thecavities702a,702b,702c, and702d, respectively; and
• (3) a plurality ofanchors703a,703b,703cand703dsurrounded by thecavities702a,702b,702c, and702d, respectively.
• With these cavities, connectors and anchors, various formations of cantilevers are possible, in similar ways discussed above.
• FIG. 8 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a third example of an addressable cMUT element in accordance with the present invention. Theaddressable cMUT element800 has an overall square shape, and includes:
• (1) an outercircular cavity802asurrounding an outercircular connector830a;
• (2) a plurality of extendedstraight cavities802bsubstantially parallel to each other;
• (3) a plurality of extendedstraight connectors830bsubstantially parallel to each other and placed over therespective cavity802b; and
• (4) a plurality of extendedstraight anchors803bparallel to each other and alternating withcavities802btherebetween.
• FIG. 9 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fourth example of an addressable cMUT element in accordance with the present invention. Theaddressable cMUT element900 has an overall square shape, and includes:
• (1) an outercircular cavity902asurrounding an outercircular connector930a;
• (2) a first plurality of extendedstraight cavities902bsubstantially parallel to each other;
• (3) a first plurality of extendedstraight connectors930bsubstantially parallel to each other and placed over therespective cavity902b;
• (4) a second plurality of extendedstraight cavities902csubstantially parallel to each other; and
• (3) a second plurality of extendedstraight connectors930csubstantially parallel to each other and placed over therespective cavity902c;
• The second plurality of extendedstraight cavities902cintersects with the first plurality of extendedstraight cavities902bto form a grid of cavities defining an array of discrete islands (anchors)903. The resultant cavity formation is a multiple small square-shaped circular cavities.
• FIG. 10 shows a schematic of the surface pattern of the cavities, anchors and connectors under the top plate layer in a fifth example of an addressable cMUT element in accordance with the present invention. Theaddressable cMUT element1000 has an overall square shape, and includes:
• (1) an outercircular cavity1002asurrounding an outercircular connector1030aand acontiguous anchor1003;
• (2) a plurality ofsimple cavities1002bseparated from each other and arranged side-by-side; and
• (3) a plurality ofdiscrete connectors1030bplaced over a respective simple cavity1002.
• In this configuration, plurality of simple anddiscrete cavities1002band plurality ofdiscrete connectors1030bshare a commoncontiguous anchor1003.
• It should be noted that although the exemplary formations inFIGS. 6-10 all have an outer circular cavity defining the boundary or perimeter of the addressable cMUT element, such an outer circular cavity is not required. This may be illustrated using the next example with reference toFIG. 11.
• FIG. 11 is a cross-section view of a cMUT structure showing acomplete cMUT element1100 and parts of neighboringcMUT elements1100A and1100B, one on each side. ThecMUT element1100 is very similar to thecMUT element500 inFIGS. 5A and 5B, except that the separations that form the cMUT elements (1100,1100A and1100B) are at different locations on the substrate than that inFIGS. 5A and 5B. Like the cMUT structure inFIGS. 5A and 5B, the cMUT structure inFIG. 11 is built on asubstrate wafer1101 and also has amiddle spring layer1120 and atop plate1140. ThecMUT elements1100,1102 and1104 are separated byseparation trenches1115 formed through thetop plate1140 and themiddle spring layer1120. Because theseparation trenches1115 are cut through transducing spaces1160 instead of connectors1130 (as is the case inFIGS. 5A and 5B), the boundary of thecMUT element1100 at the substrate wafer level is half of theanchors1103 instead of the cavities as in thecMUT element structure500.
• Furthermore, the exemplary formations inFIGS. 6-10 all have an inclusive circular connector surrounding the bulk of the cMUT element. Such a circular connector at an outer peripheral of the cMUT element may be used to seal the cMUT element during fabrication. Sealing may be necessary for applications in a liquid medium but may not be necessary for applications in air or vacuum. In any event, an inclusive circular connector at an outer peripheral is not required.
• As shown in the above examples, wide varieties of formations are available within the spirit of the present invention. Islands may be formed on the substrate wafer by forming cavities. Although in principle not all islands must serve as an anchor to form a cantilever, it is preferred that as many as islands are effectively utilized as anchors to optimize the performance and design freedom.
• As shown in the above examples, multiple cantilevers may be formed in the cantilever-forming area. The middle spring layer may have a plurality of connectors connecting to the top plate layer. The plurality of connectors may be distributed across the device element area to define a connector density profile indicating an average connector density per area at each location on the middle spring layer. Each connector and its corresponding cantilever may be considered as a spring having a spring strength, which when combined with the respective average connector density at the location can define an effective spring intensity profile. For example, the effective spring intensity may be defined as spring strength per unit area at a location, which would be determined by combining the spring strength of individual cantilevers and the number of cantilevers in a unit area (spring density).
• As shown in the examples described herein, the anchors (e.g., sidewall anchors503) and their corresponding connectors may be distributed across the substrate wafer in any pattern to achieve desired support characteristics such as device frequency responses and mode shapes.
• FIGS. 12A-12J show examples of spring (cantilever) distribution profiles. These examples also combine spring distribution profiles with special shapes and patterns of the top plate layer.
• FIG. 12A shows acMUT element1200A having a plurality of identical cantilevers each represented by aspring symbol1270 evenly distributed over the device element area. The resultant effective spring intensity is substantially uniform over the device element area.
• FIG. 12B shows anothercMUT element1200B having two types of cantilevers represented bysprings1270aand springs1270b.Springs1270buse thicker spring symbols to represent stronger spring strength. In this example, the effective spring intensity is significantly higher at area locations near a center of the device element area than locations near a perimeter of the device element area.
• FIG. 12C shows anothercMUT element1200C having two types of cantilevers represented bysprings1270aandstronger springs1270b. In this example, the effective spring intensity is lower at area locations near a center of the device element area than locations near a perimeter of the device element area. In addition, thesprings1270abar non-uniformly distributed in the device element area.
• FIG. 12D shows anothercMUT element1200D having two types of cantilevers represented bysprings1270aandstronger springs1270b. In this example, the device element area comprises afirst region1210aand asecond region1210b. The thickness of thetop plate layer1240D is greater in thefirst region1210athan in thesecond region1210b. The thickness in thefirst region1210amay correspond to a normal thickness of thetop plate layer1240D and the smaller thickness in thesecond region1210bmay correspond to a bottom thickness of a cavity formed on thetop plate layer1240D.
• FIG. 12E shows anothercMUT element1200E having a plurality of cantilevers represented bysprings1270. In this example, thetop plate layer1240E has a thickness profile having twodifferent thicknesses1212aand1212balternating across the device element area. The greater thickness may correspond to a normal thickness of thetop plate layer1240E while the smaller thickness may correspond to a bottom thickness of a cavity formed on thetop plate layer1240E. The twothicknesses1212aand1212bmay regularly alternate across the device element area. This configuration may help to improve the rigid/mass ratio of the top plate.
• FIG. 12F shows anothercMUT element1200F having a plurality of cantilevers represented bysprings1270. In this example, thetop plate layer1240F is made of two different materials. The first material may make up a plurality ofsegments1241 divided byseparation trenches1245. Thesecond material1242 may be provided as connectors, which connect thesegments1241 by spanning over thetrenches1245. Thesecond material1242 may be a sealing material to fill or partially fill thetrenches1245.
• FIG. 12G shows anothercMUT element1200G having a plurality of cantilevers represented bysprings1270, which are uniformly distributed across the cMUT area. In this example, thetop plate layer1240G has a thickness profile in which thetop plate layer1240G is thicker in a central region than in a region close to the perimeter of thecMUT element1200G.
• FIG. 12H shows anothercMUT element1200H which is similar to thecMUT element1200G except that thecMUT element1200H has two types of cantilevers represented bysprings1270aandstronger springs1270b. Thestronger springs1270bare deployed at edges (perimeter) of thecMUT element1200H.
• FIG. 12I shows another cMUT element1200I having a plurality of cantilevers represented bysprings1270 which are uniformly distributed in a middle region of the cMUT area. In this example, the top plate layer1240I has a thickness profile in which thetop plate layer1240G is significantly thinner in the perimeter of the cMUT element then in the middle region. In one embodiment, the top plate layer1240I is very thin and flexible near the parameter but rigid in the middle region. The top plate layer1240I may even be clamped (or fixed) or nearly clamped at perimeter sidewalls1281 without the support of a cantilever (spring). Even in this special case, however, there are no clamped (or fixed) areas within the surface of cMUT elements except for the edges, and the cMUT element1200I still has at least a central portion of the top plate1249I supported by the embedded springs located underneath to move in the piston-like motion. Therefore, the cMUT in such special configurations would still benefit from the potential advantages of the present invention over the conventional cMUTs.
• FIG. 12J shows anothercMUT element1200J which is similar to the cMUT element1200I except that the springs supporting the middle region of thetop plate layer1240J has two types of different springs strengths and distributed with a desired distribution pattern.
• Many other possible cMUT designs for the top plate and embedded springs may be made with the present invention.
• Taking advantage of the design flexibility in the present invention, the transducer can be made to have a desired frequency response and optimal transmission and reception performances. In addition to the variations of cantilever distribution profiles and the material selection, mass and thickness profiles of the top plate layer as shown above, other features or variations of embodiments may be incorporated into the micro-electro-mechanical transducer in accordance with the present invention.
• There are many possible combinations of material properties and size, shape and locations of the components in the micro-electro-mechanical transducer in accordance with the present invention. Electrically, any combination of the material and complement selections desirably should prevent the electrical shorting between two cMUT electrodes. The combination desirably should also enhance the electrical field between the two electrodes for a given input voltage and reduce the parasitic capacitance of the cMUT. It is also important to select materials with better reliability. For example, the mechanical reliability of the middle spring layer is important since this layer bears the maximum mechanical stress of the device. For this consideration, single crystal silicon is a good selection. In addition, the conductivity of the middle spring layer may be designed to be very low to reduce the parasitic capacitance. Therefore, a dielectric material (e.g., silicon nitride) is suitable. If single crystal silicon is selected as the material for the middle spring layer, low-doping level or intrinsic silicon may be used as a starting material for the middle spring layer, which may be subsequently selectively and highly doped in areas designed to be the bottom electrode.
• FIG. 13 shows an example of doping profile of a silicon middle spring layer. Similar to other MUT structures shown herein, the MUT structure inFIG. 13 has cavity1302 (which can be vacuum sealed in immersion application),sidewall anchor1303, plate-spring connectors1330, andtop plate layer1340. The MUT inFIG. 13, however, has a silicon middle spring layer that has two different types of regions, which are intrinsic or very lightly dopedsilicon regions1320aand highly dopedsilicon region1320b. The highly dopedsilicon region1320bmay be effectuated as a bottom electrode to substitute a separate conductive layer deposited on the middle spring layer as the bottom electrode as shown in other embodiments. Alternatively or additionally, the bottom electrode may be effectuated by using aconductive substrate wafer1301.
• Moreover, regardless of how the bottom electrode is fabricated or effectuated, the doping types can be made different betweenregions1320aand1320bin the middle spring layer ofFIG. 13 to form a PN junction between the two regions. This may further increase the breakdown voltage of the transducer and reduce the parasitic capacitance by selecting a proper bias voltage on the PN junction.
• In the cMUT structure of the present invention, the top electrode can be located in the any position as long as it moves with the top plate. However, the top electrode desirably should be designed to maximize the electric field between two electrodes without electric shorting. For example, if the top plate is conductive (e.g. highly doped silicon), the top electrode (usually a metal layer) may be place on the top of the top plate. If the top plate is not conductive (e.g., using silicon nitride, LTO, or PMMA), the top electrode may be placed at the bottom side of the top plate layer, and preferably on the top of an insulation layer (e.g.,insulation layer535 inFIG. 5B) which is placed over the plate-spring connectors. This helps to obtain a stronger electrical field between two electrodes.
• One highly effective method for forming or introducing top plate in a cMUT structure of the present invention is to use wafer-bonding technology in which a prime silicon wafer or an SOI wafer having a desired silicon layer on an insulation layer is bonded to the substrate and the middle spring layer. In that case, the resultant top plate would include a silicon layer or polysilicon layer. Such a layer is relatively conductive for the purpose of the applications envisioned in the present invention. Correspondingly, measures to reduce the chance for shorting between the top electrode and the bottom electrode may be used.
• An insulation layer such as theinsulation layer535 inFIG. 5B is used to prevent shorting between two electrodes in case the two electrodes contact each other during the operation. The insulation layer can be located on either the top or the bottom side of the transducing space defined between two electrodes. Theinsulation layer535 inFIG. 5B is an example of the former configuration. Usually it is preferable to have the insulation layer placed on the side of the top plate instead of having it on the middle spring layer. This is because the properties of the middle spring layer needs to be well controlled, and having an insulation layer thereon may impose an unfavorable restriction for such controlling of the properties and behaviors. The insulation layer may either be a contiguous single layer or be patterned. If the middle spring layer itself is made of a dialectical material (e.g., silicon nitride), the insulation layer may be optional.
• The primary function of an insulation layer such as theinsulation layer535 is to prevent the shorting between two the electrodes during the cMUT operation. However, using an insulation layer may introduce drawbacks. First, it may trap some charges, which may degrade the reliability of the transducer. Second, there exists a voltage drop cross the insulation layer and this voltage drop may reduce the transducer efficiency, especially for a cMUT (e.g. a high frequency cMUT) that needs a small vertical displacement and a high input voltage.
• According to one aspect of the present invention, the above drawbacks may be overcome by eliminating the need for theinsulation layer535 using special designs to perform an equivalent function of the insulation layer.
• In one design, the depth (vertical height) of the cavity in the substrate wafer is designed to be no greater than, preferably smaller than, the depth (vertical height) of the available transducing space between the top plate and the middle spring layer. In this design, the middle spring layer would be stopped by the bottom of the cavity in the substrate wafer before the conductive top plate and the top electrode hit the bottom electrode to cause shorting between two electrodes. This would effectively prevent the top plate layer from touching the bottom electrode to cause a short, and thus eliminate the need for an insulation layer under the top plate layer.
• If a large depth is desired for the cavity below the middle spring layer, a motion stopper may be introduced in the cavity to achieve a similar function as described below with reference toFIG. 14.
• FIG. 14 shows an enlarged view of a selected portion of a cMUT element having a motion stopper to limit the maximum vertical displacement of the top plate. The selectedcMUT portion1410 is similar to the selectedcMUT portion510 inFIG. 5 but hasmotion stoppers1490 to prevent shorting. The selectedcMUT portion1410 has two halves centered on an imaginary vertical line (not shown) going through the structure from the top to the bottom. The basic structure of the cMUT element is built on asubstrate wafer1401 and has these components: acavity1402 defined by asidewall anchor1403, amiddle spring layer1420 which is preferably an elastic membrane, abottom electrode1425 placed on the middle spring layer1400, aconnector1430 which stands on top of themiddle spring layer1420, atop plate1440, and atop electrode1450. Unlike the cMUT structure inFIGS. 5A-5B, there is no intervening insulation layer between thetop plate layer1440 and theconnector1430.
• Theconnector1430 stands out from themiddle spring layer1420 to define atransducing space1460 below thetop plate layer1440. In this particular embodiment, the actual height D_aof thetransducing space1460 is reduced by the thicknesses of thebottom electrode1425 and themiddle spring layer1420 in the configuration shown inFIG. 14. Theconnector1430 is horizontally distanced from thesidewall anchor1403 by a sufficient length to define a cantilever anchored at thesidewall anchor1403. The cantilever and thecavity1402 enable a vertical displacement of theconnector1430, which transports thetop plate1440 substantially vertically with a piston-like motion, thus changing thetransducing space1460. When the both halves of thecMUT structure1410 move in the same phase, the vertical piston-like motion is further assured.
• As shown inFIG. 14, the maximum vertical displacement D_mof theconnector1430 is limited by amotion stopper1490 disposed in thecavity1402. When D_mis designed to be no greater than (preferably smaller than) D_a, the vertical displacement of the connector1430 (and thus the maximum vertical transportation distance of the top plate layer1440) is limited to be less than the height D_aof the transducing space. This effectively prevents thetop plate layer1440 from touching thebottom electrode1425 to cause a short, thus eliminating the need for an insulation layer under thetop plate layer1440. In one preferred embodiment, D_mis at least one third less than D_a.
• Thestopper1490 can be placed at different locations with a desired height. In addition, as in the other designs described herein, thebottom electrode1425 is optional if thesubstrate wafer1401 and/or at least a part of the middle spring layer is conductive.
• In general, if the substrate wafer and the middle spring layer are conducted, these layers should preferably be connected together electrically (e.g. to have the same electric potential). Alternatively, either the substrate wafer or the middle spring layer, or both, may be made of insulation materials.
• Ideally, the top plate should be light (i.e., with less mass) and rigid. An acoustic ratio of 1st resonant frequency over the total mass, defined as a rigidity/mass ratio, of the top plate may be used to evaluate the material selection and structure design of the top plate. A higher value of the rigidity/mass ratio is generally preferred for the top plate. Several different designs considerations are described below.
• (1) If the top plate is made of a solid plate of a single material, the selected material itself should have a low mass density and high Young's modulus (e.g., diamond is one of the best materials for such a purpose).
• (2) If a certain material is already selected for the top plate, some structure designs may be used to further improve the rigidity/mass ratio. For example, making a material porous usually increases the rigidity/mass ratio defined above. Porous silicon can be formed by electrochemical etching of silicon in an HF rich electrolyte. For another example, the top plate can be micromachined to have a well-designed hollow structure to achieve a larger ratio of 1st resonant frequency over the total mass. The micromachining may be done by etching with a desired mask pattern.
• (3) For a given material, certain treatments can be introduced to increase the equivalent Young's modulus of the materials without increasing the mass. One possible way to do this is to introduce a tension stress in the materials. For example, the stress can be introduced in a thin film formation by controlling the process parameters; the stress can also be introduced into silicon by proper doping if the silicon is used.
• The above considerations and methods may be also applied to the materials of the middle spring layer.
• Both the top plate and the middle spring layer may be micromachined structures such as a porous structure to achieve modified mechanical properties. For example, the top plate may be made of plates with holes built therein. Alternatively, the holes may be half-through cavities formed on top of the top plate without penetrating the entire thickness thereof. Hollow structures such as voids may also be formed within the top plate with no openings or only very small openings on the surface of the top plate.
• FIGS. 15A-15D illustrate examples of holed or hallowed structures applied to the top plate to optimize the properties such as the rigidity/mass ratio of the top plate.
• FIGS. 15A-15C show three exemplary configurations of a top plate layer of the present invention.FIG. 15D shows a graph of the corresponding ratio of 1st resonant frequency over the total mass of the top plate as the function of the diameter of the etched holes in the three configurations. In the first configuration shown inFIG. 15A, an array ofholes1544A of a diameter of 8 μm is formed on thetop plate1540A. There is a separation distance of 10 μm between the neighboring holes. In the second configuration shown inFIG. 15B, an array ofholes1544B of a diameter of 4 μm is formed on thetop plate1540B. There is a separation distance of 10 μm between the neighboring holes. In the third configuration shown inFIG. 15C, a solid top plate1540C without holes formed therein is used.
• As shown in the graph ofFIG. 15D, the first configuration shown inFIG. 15A is a preferred top plate design because it demonstrates the highest rigidity/mass ratio. In these figures, the results of the ratios for three different silicon structure designs for the top plate are normalized by that of a solid silicon plane.
• The hollow structures shown inFIGS. 15A-15C are just some examples. It is appreciated that many other possible hollow structure patterns (e.g., hexagon, square, or triangle), with the different shapes, sizes, densities and configurations of the trenches, can be used for the purpose of the present invention.FIG. 15E, for example, shows another example of the top plate having holes built therein. Thetop plate1540E hasholes1544E that are built from a side instead of a major surface (e.g. the top surface) of thetop plate1540E.
• If needed, the hollow structure can be designed such that the holes may be refilled or sealed using materials with desired properties (e.g., a light and stiff material).
• In general, it is preferred that the top plate is significantly more rigid than the middle spring layer and is substantially unbent when transported by the vertical displacement of the connector(s). For example, if the same or similar materials are used for both the top plate layer and the middle spring layer, the top plate layer may desirably be at least three times as thick as the middle spring layer, and more preferably at least five times as thick.
• In a capacitance micromachined ultrasonic transducer (cMUT), the bottom electrode may be either on the middle spring layer or on the substrate wafer. In the present description, the term “on” does not necessarily suggest that a separate material or layer is placed on another layer. The bottom electrode may be a part of the middle spring layer or the substrate wafer. For example, the middle spring layer may comprise a conductive material to effectuate the bottom electrode.
• In one embodiment, multiple addressable device elements are fabricated on the same substrate wafer. If the substrate wafer is a conductive wafer, the multiple addressable device elements may be insulated from each other by an embedded insulator formed on the substrate wafer. Insulators and the methods of fabricating the same as disclosed in the several patent applications identified herein filed by the common applicant on even date may be useful this purpose. For example, the embedded insulator may have an insulation cavity having a desired overall thickness and being filled with a dielectric material. The embedded insulator may alternatively have a patterned cavity containing solid lines of oxidized native material of the substrate wafer. Similar embedded insulators may be formed on the top plate layer.
• The micro-electro-mechanical transducer of the present invention may use various addressing schemes to address the addressable device elements described herein. In particular, the addressable device elements may be addressed from back side of the substrate wafer via a through-wafer interconnect formed through the substrate wafer, as disclosed in the several patent applications identified herein filed by the common applicant on even date. For example, a conductive substrate wafer may be used, and the through-wafer interconnect having an annular trench surrounding a through-wafer conductor made of a native material of the conductive wafer may be used for interconnecting the multiple transducer elements. The through-wafer interconnect may further comprise a dielectric material filled into the annular trench to insulate the through-wafer conductor from rest of the substrate wafer. In one embodiment, the through-wafer conductor is positioned directly underneath the addressable device element.
• Although the micro-electro-mechanical transducer of the present invention has been illustrated mostly using a cMUT as an example, other types of transducers such as pMUT and mMUT can also be formed using the same concept in accordance with the present invention.
• FIG. 16 shows the cross-section view of a section of a pMUT element according to an exemplary embodiment of the present invention. The pMUT is similar to the cMUT illustrated herein, and hassubstrate wafer1601,cavities1602, sidewall anchors1603,middle spring layer1620, plate-spring connectors1630,top plate1640, andpiezoelectric members1675 placed on themiddle spring layer1620 in atransducing space1660. Unlike cMUT, the pMUT structure does not have electrodes to form a capacitor having a static electrode and a movable electrode. Instead, apiezoelectric member1675 function as a transducing member in combination with the cantilevers formed from the resilientmiddle spring layer1620. Eachpiezoelectric member1675 may include two electrodes sandwiching a piezoelectric layer therebetween. Despite the similarities in the overall structure, the material selection may be different for pMUT because the electric connections in the pMUT transducer are different due to a different energy transformation method.
• FIG. 17 shows the cross-section view of a section of amMUT element1710 according to an exemplary embodiment of the present invention. The mMUT is similar to the cMUT and pMUT illustrated herein, and hassubstrate wafer1701,cavities1702, sidewall anchors1703,middle spring layer1720, plate-spring connectors1730,top plate1740, transducingspace1760, andmagnetic members1775 placed on thetop plate layer1740. Unlike cMUT, the mMUT structure does not have electrodes to form a capacitor.Magnetic members1775 function as a transducing member or energy exchanger activating the top plate in response to the substantially vertical motion of thetop plate layer1740 caused by the cantilevers formed out of the resilientmiddle spring layer1720. Despite the similarities in the overall structure, the material selection may be different for mMUT because the electric connections in the mMUT transducer are different due to a different energy transformation method.
Integration of MUT and Integrated Circuits
• The basic structure of the micro-electro-mechanical transducer described herein may be integrated with integrated circuits (IC).FIG. 18 shows a cross-section view of adevice1800 having acMUT1810 integrated with an IC. ThecMUT1810 includessubstrate wafer1801, gaps orcavities1802, sidewall anchors1803,middle spring layer1820, plate-spring connectors1830, a movabletop plate1840, atop electrode1850, atransducing space1860, abottom electrode1825, and one or moreintegrated circuits1849 built in thetop plate1840. Metallization may be used to form thetop electrode1850 of thecMUT1810 and to connect nearby cMUT elements withICs1849.
• Since thetop plate1840 can be made of single crystal silicon with desired properties (e.g. orientation, doping level and type and the thickness, etc.), integrated circuits (IC)1849 can be fabricated directly on thetop plate layer1840 to accomplish the integration as shown inFIG. 18. TheICs1849 may be fabricated right after the formation or placement of the top of theplate1840. At that stage of the process, the surface of the fabricated wafer with embedded transducers is the bare surface of thetop plate layer1840, which is almost the same as that of a prime wafer. Therefore,ICs1849 can be fabricated by the standard IC fabrication methods. The process after theICs1849 have been fabricated should be carefully chosen so that the ICs will not be damaged by the subsequent process (e.g., temperature and etchant). Usually, after forming or placing thetop plate1840, no high-temperature process may be required to complete the fabrication of the transducer in order to make the integration feasible.
• TheICs1849 may be fabricated on the top of thecMUT top plate1840 within the device element area as shown, or fabricated in an adjacent area on thetop plate1840 outside of the device element area (not shown), such as an area between two device elements reserved for IC fabrication.
• Alternatively, a wafer with desired ICs preformed can be introduced and bonded with the cMUT wafer during the transducer fabrication to achieve integration. The process after introducing the ICs wafer should be carefully selected to avoid of damaging ICs during the rest process steps.
• The micro-electro-mechanical transducer of the present invention may be fabricated using the fabrication methods described in the several patent applications identified above filed by the common applicant on even date. The fabrication methods for making the basic micro-electro-mechanical transducer of the present invention include (1) Fabrication using wafer-bonding technology; (2) Fabrication Using sacrificial technology; and (3) Fabrication using a combination of wafer-bonding and sacrificial technologies.FIGS. 19-25, from U.S. Provisional Application No. 60/682,619, previously incorporated herein by reference, are discussed below and further illustrate and describe techniques for incorporating ICs with cMUTs and/or incorporating multiple cMUTs with one another.
• FIGS. 19A-19D illustrate one example of the fabrication acMUT device1900 having ICs integrated with acMUT transducer element1910.FIG. 19A shows cMUT plane layer formation, such as by deposition of atop plate layer1940 ontoconnectors1930 formed on a membrane ormiddle spring layer1920 to form atransducing space1960. As discussed above, themiddle spring layer1920 may be formed on an etchedsubstrate wafer1901 havingcavities1902 and sidewall anchors1903. After the formation of thetop plate layer1940, the wafer surface properties of thetop plate layer1940 may be the same as theprime wafer1901. Ananchor area1938 may also be included, such as for a wire bonding pad or transducer interface connection.
• FIG. 19B illustrates that desired integrated circuits (ICs)1949 can be fabricated with a standard IC process on thetop plate1940.FIG. 19C illustrates that metallization may be used to form atop electrode1950 of thecMUT element1910 and to connect thecMUT element1910 with theICs1949. Thus, during formation of thetop electrode1950, interconnections with the ICs may be formed. In some cases, a through-wafer via process may be used to access thetop electrode1950 from underneath the top plate1940 (not shown in this example).
• Next, as illustrated inFIG. 19D, thecMUT transducer element1910 may be separated by an etching process through the formation ofseparation trenches1915, as discussed above. Other methods to integrate the cMUT transducer elements and ICs are shown and described in U.S. Provisional Application No. 60/682,619, previously incorporated herein by reference, including various other details and alternatives for the fabrication process. For example, a wafer with desired ICs can be bonded with the cMUT wafer during transducer fabrication. As discussed above, the processing of thecMUT device1900 after the IC wafer is added may be carefully selected to avoid damaging the ICs during the rest of the processing steps.
• CMUTs with Separated Transmission and Reception Capacitors
• FIGS. 20A and 20B illustrate electrical connections for cMUTs that may be used for both transmission and detection (reception) operations according to some implementations.FIG. 20B illustrates an example cMUT arrangement with electrical connection for a cMUT device that has two cMUTs able to be used separately for transmission and detection operations, respectively.
• The basic structure of the cMUT may be modeled as a variable capacitor. In the transducers designed for most applications, the same cMUT may be used for both transmission and reception of acoustic energy, as shown inFIG. 20A.FIG. 20A illustrates a single cMUT2010 (represented schematically as a variable capacitor) that may be used alternately for both transmission and reception of acoustic energy.CMUT2010 is connected on one side to a bias circuit or desiredbias voltage source2012.CMUT2010 is connected on the other side to aswitch2014.Switch 2014 may connectcMUT2010 to either atransmission interface circuit2016 or areception interface circuit2018. There may be a few drawbacks to this approach such as the use of aswitch circuit2014 to protect the preamplifier of thereception circuit2018. However, even with aswitch circuit2014, the transmission signal may still couple to the reception circuit side through some parasitic capacitance and may affect the performance of thereception circuit2018. In addition, since both transmission and reception operations use thesame cMUT2010, the overall device performance may be a trade-off between transmission and reception performance. Accordingly, for cMUT transducers in some applications, it may be desired that transmission and reception operations use two separate cMUTs, as shown inFIG. 20B.
• FIG. 20B illustrates acMUT device2030 with a first cMUT2031 (cMUT1) and a second cMUT2032 (cMUT2), one of which is used for transmission and the other of which is used for reception.CMUTs2031 and2032 may be connected to a bias circuit or a desiredvoltage source2012 through amiddle electrode2037. In the illustrated example,first cMUT2031 is dedicated to transmission throughtransmission interface circuit2016 andsecond cMUT2032 is dedicated to reception throughreception interface circuit2018. One advantage of the arrangement ofFIG. 20B includes that theswitch circuit2014 is no longer needed to protect a preamplifier of thereception circuit2018. For example, the electric signal from transmission may be shielded by thebiased electrode2037 in the middle of thedevice2030 so that cross-talking between transmission and reception is minimized. Therefore, the front-end reception circuit2018 can be optimized without worrying about saturation from coupling of the transmission signal. Thus, thefirst cMUT2031 may be separately operable from thesecond cMUT2032, e.g., thefirst cMUT2031 may be operated in transmission and thesecond cMUT2032 may be operated in reception.
• In addition, because the transmission and reception operations usedifferent cMUTs2031 and2032, respectively, the device design does not need to make a trade-off between transmission and reception performances. Accordingly, both transmission and reception performances can be optimized at the same time, which may dramatically improve the whole device performance of thecMUT device2030. For example, thereception cMUT2032 may have a very thin insulation layer and a very narrow gap so that a low DC bias voltage is required and the capacitance of thereception cMUT2032 can be increased significantly. This may dramatically improve the cMUT sensitivity during reception.
• FIGS. 21A and 21B illustrate schematics of transducer devices that may use different cMUTs for transmission and detection, respectively. There are two cMUTs in each transducer device, such that a first cMUT may be used for transmission and a second cMUT may be used for reception.FIG. 21A illustrates atransducer device2100 using different cMUTs for transmission and reception by placement of two cMUTs side-by-side, as a first cMUT2102 (cMUT1) and a second cMUT2104 (cMUT2) located on asurface2106. Compared with a transducer device using the same cMUT for both transmission and reception, one disadvantage associated with this approach includes that thecMUTs2102,2104 occupy a larger area. This is particularly undesirable for a transducer with a large number of elements. Also, since the cMUTs for transmission and reception are located at different surface positions, and moreover, may have different shapes or patterns, these differences may result in the different beam patterns between transmission and reception operation, and may have some other impact on the device performance (such as the accuracy of ranging and imaging) and/or may require extra calibration efforts.
• FIG. 21B illustrates anexample device2110 in which a first cMUT2112 (cMUT1) is positioned directly on a second cMUT2114 (cMUT2) in a stacked relationship according to some implementations. Therefore, the device shapes and patterns of bothcMUTs2112,2114 can be designed to be identical. In addition, the locations of the twocMUTs2114,2114 are the same on thetransducer device surface2116 with a slightly different depth, whose effects usually can be ignored because it is much shorter than the ultrasonic wavelength around the operation frequency. There may be many transducer device configurations and associated fabrication methods to achieve the design concept included in the design ofFIG. 21B. Several possible examples are described below.
• FIG. 22A illustrates a cross section of acMUT device2200 with a first cMUT2211 (e.g., for transmission) and a second cMUT2212 (e.g., for reception) in a stacked relationship according to some implementations. ThecMUT device2200 includes asubstrate wafer2201, gaps or cavities2202, a support oranchor2203, a membrane layer orspring member2220, plate-spring connectors or supports2230, anoptional insulation layer2235 and amovable plate2241. Thedevice2200 may further include acommon middle electrode2237 that is part of bothcMUTs2211,2212. Thecommon electrode2237 serves as the top electrode of thefirst cMUT2211 and as the bottom electrode of thesecond cMUT2212. Thedevice2200 further includes abottom electrode2225 of the first cMUT (which may be optional if thesubstrate2201 and/or the part of themembrane2220 are conductive). Thedevice2200 further includes a gap or cavity as a first transducing space2261 (which may be vacuum sealed in immersion applications) for thefirst cMUT2211. Thedevice2200 further includes supports or anchors2232 to support a flexible element ormembrane2222 of thesecond cMUT2212. The flexible element ormembrane2222 of thesecond cMUT2212 may include an insulation layer (not shown) and may be supported away from theplate2241 by thesupports2232 to form a second gap or transducing space2262 between themembrane2222 and the middle electrode2237 (which may be vacuum sealed in immersion applications). Atop electrode2250 serves as the top electrode for thesecond cMUT2212.
• The example ofFIG. 22A is based on a cMUT with an embedded spring member underneath the middlemovable plate2241. Thesecond cMUT2212 may be built on the top of themovable plate2241 of thefirst cMUT2211. The movable plate2241 (or the common middle electrode2237) may serve as the ground or bias electrode. Therefore, the plate2241 (or electrode2237) can shield from coupling between a transmission signal and a reception signal during the device operation.
• The fabrication process of thecMUTs2211,2212 may include fabricating the cMUTs twice at the same wafer location. For example, afirst cMUT2211 having a desired design may be fabricated first, and then asecond cMUT2212 having a desired design may be fabricated on the top of thefirst cMUT2211, and bothcMUTs2211,2212 may share one common electrode (e.g.,electrode2237 inFIG. 22A or optionallymovable plate2241 in the case that themovable plate2241 is conductive).
• FIG. 22B illustrates the cross-section view of thecMUT device2200 having thefirst cMUT2211 integrated with thesecond cMUT2212 and further includingintegrated ICs2249 according to some implementations. For example, if the material (e.g., a single crystal silicon layer with proper doping level and thickness) of themovable plate2241 of thefirst cMUT2211 is properly chosen, integrated circuits (ICs)2249 may be fabricated onplate2241 after formation of thefirst cMUT2211, as shown inFIG. 22B. Thesecond cMUT2212 may then be fabricated on the top of theICs2249. In this example, a front-end circuit (not shown) may be positioned very close to thetransducer device2200 so that noise and the parasitic capacitance may be dramatically reduced. Therefore, the cMUT performance may be further improved.
• FIG. 23A illustrates a cross-section view of acMUT device2300 with separate cMUTs including afirst cMUT2311 integrated with asecond cMUT2312 in a stacked relationship according to some implementations. ThecMUT device2300 includes asubstrate wafer2301, gaps orcavities2302, a membrane orspring member2320, plate-spring connectors or supports2330, anoptional insulation layer2335, amovable plate2340 and atop electrode2350. Thetop electrode2350 is optional if theplate2340 is conductive. Thedevice2300 may further include a common electrode2337 disposed on thespring layer2320 that is part of bothcMUTs2311 and2312. The common electrode2337 serves as the top electrode of thefirst cMUT2311 and as the bottom electrode of thesecond cMUT2312. An anchor orsupport2303 may separate themembrane2320 from thesubstrate2301. Thedevice2300 further includesbottom electrodes2325 of the first cMUT2311 (which may be optional if thesubstrate2301 is conductive and thesupport2303 is non-conductive). Thedevice2300 further includes a gap or cavity2260 as a transducing space of thesecond cMUT2312, while the gaps orcavities2302 serve as the transducing space of the first cMUT2311 (both of which may be vacuum sealed in immersion applications).
• The example ofFIG. 23A is also based on a cMUT with embedded spring member underneath theplate2340. In this example, thesubstrate2301 is made of insulation material (e.g. glass, sapphire, silicon covered by an insulation layer, etc.) so that a patterned conductive layer can be placed on thesubstrate2301 as the bottom electrode(s)2325 of thefirst cMUT2311. Thesubstrate2301 can be also replaced by the PCB board with a desired circuit and a wafer with desired ICs and metal patterns. Fabrication techniques are further shown and described in U.S. Provisional Application No. 60/682,619, previously incorporated herein by reference.
• FIG. 23B illustrates a cross-section view of thecMUT device2300 ofFIG. 23A, further incorporatingintegrated circuits2349 according to some implementations. The integrated circuits can be fabricated on theplate2340, as described above with respect toFIGS. 19A-19D, if theplate2340 is made of silicon with desired properties.
• The design and fabrication of connections and interconnections disclosed in the several patent applications identified herein may also be used, in addition to any other available designs and methods of interconnections.
• The trenches between the micro-electro-mechanical transducer elements may be sealed in some applications. For example, a sealing structure may help prevent the medium or moistures leaking into the trench. A sealing structure may also be designed to minimize coupling between elements and to provide electrical connections between the elements. The trenches may be sealed either during or after the device fabrication. Techniques for sealing trenches between the device elements disclosed in the several patent applications identified herein may be used.
• The micro-electro-mechanical device such as the cMUT of the present invention may need to interconnect to the outside through bonding wires or the probes. To do this, electrical interface pads (e.g., wire-bonding pads or probing pads) may be needed. The interface pads usually would introduce some undesired parasitic parameters (e.g., parasitic capacitance, or inductance). In order to improve the transducer performance, the parasitic parameters may need to be minimized.
• In addition to any available methods for reduction of parasitic capacitance, the novel methods to reduce parasitic capacitance of an interconnection pad by increasing the thickness of an insulator underneath the interconnection pad disclosed in the several patent applications identified herein may also be used. The method allows the thickness of insulation layer on the substrate to be defined by the etching depth instead of the thickness of a film deposition, thus making possible to fabricate insulators of a thickness significantly greater than what would be possible with film deposition techniques.
• The micro-electro-mechanical transducer in accordance with the present invention has been described in detail along with the figures and exemplary embodiments. The transducer potentially can alleviate or eliminate a number of problems with existing technology. The invention has eliminated the necessity of forming an addressable transducer element using a great number of smaller cells. Using the technology, either a much fewer cells are just a single cell may be necessary for each addressable transducer element. The design of the micro-electro-mechanical transducer of the present invention is particularly suitable for application in capacitance micromachined ultrasonic transducers (cMUT), but can also be used for other micro-electro-mechanical devices, which have a movable mechanical part to transform energy.
• In particular, the micro-electro-mechanical transducer in accordance with the present invention may be fabricated using the novel fabrication methods disclosed in international patent applications PCT/IB2006/051566, entitled THROUGH-WAFER INTERCONNECTION; PCT/IB2006/051567, entitled METHODS FOR FABRICATING MICRO-ELECTRO-MECHANICAL DEVICES; and PCT/IB2006/051569, entitled MICRO-ELECTRO-MECHANICAL TRANSDUCERS.
• In the foregoing specification, the present disclosure is described with reference to specific embodiments thereof, but those skilled in the art will recognize that the present disclosure is not limited thereto. Various features and aspects of the above-described disclosure may be used individually or jointly. Further, the present disclosure can be utilized in any number of environments and applications beyond those described herein without departing from the broader spirit and scope of the specification. The scope of the claims includes all such modifications and variations that fall within the scope and spirit of the claims below. The specification and drawings are, accordingly, to be regarded as illustrative rather than restrictive. It will be recognized that the terms “comprising,” “including,” and “having,” as used herein, are specifically intended to be read as open-ended terms of art.

Claims (28)

1. A device comprising:

a first capacitive micromachined ultrasonic transducer (cMUT); and

a second cMUT contiguous with the first cMUT in a stacked relationship.

2. The device according toclaim 1, wherein at least one of the first cMUT or the second cMUT includes at least one integrated circuit formed on at least one surface.

3. The device according toclaim 1, wherein the first cMUT comprises:

a first electrode;

a second electrode separated from the first electrode by a first gap maintained by one or more first supports; and

a spring member for enabling the first electrode and the second electrode to move toward and away from each other due to the first gap.

4. The device according toclaim 3, wherein the second cMUT comprises the second electrode and a third electrode separated from the second electrode by a second gap on the opposite side of the second electrode from the first electrode, the second gap being maintained by one or more second supports.

5. The device according toclaim 4, wherein the third electrode is associated with a flexible element that enables the third electrode to move toward and away from the second electrode due to the second gap.

6. The device according toclaim 4, wherein:

the third electrode is associated with a plate positioned over the second electrode and the spring member, and separated therefrom by a second gap maintained by the one or more second supports;

the third electrode is able to move toward and away from the second electrode; and

the third electrode is located primarily over one or more of the first supports.

7. The device according toclaim 1, wherein the first cMUT has a different operation property from the second cMUT.

8. The device according toclaim 1, wherein the first cMUT and the second cMUT share a common middle electrode and are operable separately from one another.

9. The device according toclaim 1, wherein:

the first cMUT is connected to a reception circuit to receive acoustic energy; and

the second cMUT is connected to a transmission circuit to transmit acoustic energy.

10. The device according toclaim 1, wherein a common electrode of the first cMUT and the second cMUT is connected to a bias circuit or voltage source.

11. The device according toclaim 1, further comprising:

a movable plate as part of at least one of the first cMUT or the second cMUT; and

at least one integrated circuit formed on the movable plate.

12. The device according toclaim 1, wherein the first cMUT is electrically operable separately from the second cMUT.

13. A device comprising:

a first electrode;

a second electrode separated from the first electrode by a first gap;

a spring member and one or more first supports for maintaining the first gap between the first electrode and the second electrode, wherein the first electrode and the second electrode are able to move toward and away from each other causing flexing of the spring member; and

a third electrode separated from the second electrode by a second gap on an opposite side of the second electrode from the first electrode, wherein the first electrode is connected to a first circuit and the third electrode is connected to a second circuit, different from the first circuit.

14. The device according toclaim 13, wherein:

the second electrode is a plate or is located on a plate;

the third electrode is contiguous with a flexible element that is separated from the plate by one or more second supports; and

the flexible element enables the third electrode to move toward and away from the second electrode.

15. The device according toclaim 13, wherein:

the second electrode is contiguous with the spring member;

the third electrode is separated from the second electrode and the spring member by one or more second supports; and

flexing of the spring member enables the third electrode to move toward and away from the second electrode.

16. The device according toclaim 15, wherein the third electrode is located over one or more of the first supports.

17. The device according toclaim 13, wherein:

the first electrode and the second electrode comprise a first capacitive micromachined ultrasonic transducer (cMUT); and

the second electrode and the third electrode comprise a second cMUT.

18. The device according toclaim 17, wherein the second electrode is connected to a bias circuit or voltage source, and serves as a common electrode to the first cMUT and the second cMUT.

19. The device according toclaim 17, wherein the second electrode is a common electrode shared by the first cMUT and the second cMUT, the common electrode having a bias to shield from coupling between a transmission signal received by the first cMUT and a reception signal provided by the second cMUT.

20. A device comprising:

a substrate;

a plate disposed above the substrate;

a spring member between the substrate and the plate to enable movement of the plate toward and away from the substrate; and

at least one other device formed on the plate.

21. The device according toclaim 20, wherein the at least one other device is an integrated circuit.

22. The device according toclaim 20, wherein the at least one other device is a capacitive micromachined ultrasonic transducer.

23. The device according toclaim 20, further comprising:

a first electrode associated with the substrate; and

a second electrode associated with the plate, wherein a gap between the substrate and the plate allows movement of the second electrode toward and away from the first electrode.

24. The device according toclaim 20, wherein the substrate, the plate and the spring member are included in at least one capacitive micromachined ultrasonic transducer (cMUT).

25. The device according toclaim 24, wherein:

the substrate is included in a first cMUT; and

the plate is included in a second cMUT operable separately from the first cMUT.

26. The device according toclaim 24, wherein:

the substrate and the plate are included in a first cMUT; and

at least a portion of a second cMUT is formed on the plate, the second cMUT operable separately from the first cMUT.

27. The device according toclaim 20, wherein:

a first electrode is associated with at least one of the substrate or the flexible member;

a second electrode is associated with the plate;

a flexible element is supported away from the plate on a side of the plate opposite to the substrate;

a third electrode is associated with the flexible element;

the first electrode and the second electrode are included in a first capacitive micromachined ultrasonic transducer (cMUT); and

the second electrode and the third electrode are included in a second cMUT.

28. The device according toclaim 20, wherein:

a first electrode is associated with the substrate;

a second electrode is associated with the flexible member;

a third electrode is associated with the plate;

the first electrode and the second electrode are included in a first capacitive micromachined ultrasonic transducer (cMUT); and

the second electrode and the third electrode are included in a second cMUT.

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